1. Field of the Invention
This invention relates to photography, and, more particularly, to photographic apparatus for use in cameras of the type having distinct ambient and flash exposure modes of operation to measure scene brightness and provide the photographer with a signal which alerts him to the presence of a low scene brightness condition requiring that the camera be used in its flash mode of operation.
2. Description of the Prior Art
It is generally well known among photographers that the ambient light reflected from a scene to be photographed may be insufficient to take a sharp acceptably exposed picture without using a source of artificial illumination such as a photoflash lamp or strobe light. This is true for cameras which have manual shutters as well as for those which have automatic exposure control systems which have present programs of aperture and time that vary with scene brightness.
Determining exactly when it is too dark, however, is a fairly complex process which involves a consideration of such factors as film speed, camera exposure delivery capability, and the shutter speed at which camera motion is likely to cause blurred pictures. The relationships between these important factors and the brightness of the photographic scene may be correlated with the aid of a scene light brightness measuring device (light meter) to determine when it is necessary to use an auxiliary light source to avoid underexposed or blurred pictures or both.
Those skilled in the photographic arts have recognized the problem associated with determining when there is adequate ambient scene brightness and have provided scene light measuring apparatus by which a photographer is alerted that a scene brightness condition exists which will cause exposure problems.
For example, in U.S. Pat. No. 3,810,207 issued to Arthur Z. Mueller on May 7, 1974 and entitled "Exposure Control System", the patentee discloses a camera exposure control system which provides the photographer with a visual signal in the camera viewfinder that either an overexposure or underexposure condition exists. The exposure control system of this patent includes an exposure meter having light sensing means, such as a photocell, and drive means, such as a galvanometer. The light sensing and drive means are coupled in a manner which moves the drive means in relation to the intensity of the light sensed in the field of view of the camera. A lens system is included which defines an optical axis for the camera. Iris blade means are arranged to be driven across the optical axis between selected first and second positions. The blade means define an aperture means having a configuration which varies from a minimum to a maximum area so that the amount of light through the lens system is varied as the blade moves between the first and second positions. The aperture means is also arranged to maintain the minimum and maximum exposure areas for the lens system as the meter drive moves the blades a predetermined distance beyond the established first and second positions. Suitable indicia means are coupled to the blades to register overexposure when one of the blades moves beyond the second position and underexposure when another of the blades moves beyond the first position. No provision is made for taking flash pictures, and the exposure control system is only operative when the user depresses a switch which electrically connects the system to an electronic storage battery.
Another example is found in U.S. Pat. No. 3,855,601 issued to Takashi Uchiyama et. al. on Dec. 17, 1974 and entitled "Photometer". Here, the patentees provide a camera with a photometer capable of indicating the necessity of switching from a natural light exposure mode to a flash exposure mode or of automatically switching from the natural light exposure mode to the flash exposure mode, when the brightness of a portion of the field of view containing the object to be photographed becomes lower than that of the remaining field of view by more than a predetermined difference, regardless of the general level of brightness. This apparatus is relatively complex because of all the functions and decisions it is called upon to make and, as well, is only operative in response to the actuation of a multi-step camera actuating button.
A further example is described in U.S. Pat. No. 4,007,469 issued to Edwin H. Land et. al. on Feb. 8, 1977 and entitled "Photographic Apparatus with Plurality of Selectively Determinable Operational Modes". Here, the patentees disclose a camera having a viewfinder in which there is a plurality of selectively illuminable indicators, each responsive to a different set of conditions to produce an indication that the photographer should do something else in order to produce the correct exposure. A rather complex electronic circuit is provided for sensing the state of charge of a flash unit, the ambient light level, and a pair of shutter buttons and then indicating to the photographer, via the illuminable indicators, whether or not to make a flash exposure or an ambient light exposure, or displaying an appropriate indication to the operator of what to do next. The apparatus of this disclosure is also rather complicated because of its multi-functional nature and is not designed to be continuously on to determine the state of the ambient brightness level.
Therefore, it is a primary object of the present invention to provide a simple low scene brightness indicator for use in a camera to alert a photographer of the presence of a scene brightness condition which requires the use of a flashlamp or the like to get sharp adequately exposed photographs.
It is another object of the present invention to provide a low scene brightness indicator which continuously monitors the brightness of a photographic scene without operator intervention so long as the apparatus is connected with an appropriate power source.
It is another object of the present invention to provide a low scene brightness indicator having low power consumption characteristics which enable it to continuously monitor scene brightness levels over a relatively long period of time.
Other objects of the invention will in part be obvious and will in part appear hereinafter. The invention accordingly comprises the apparatus possessing the construction, the combination of elements, and arrangement of parts which are exemplified in the following detailed disclosure.